Strongly textured atomic ridge and dot MOSFETs, sensors and filters

ABSTRACT

The present invention provides a MOSFET device comprising: a substrate including a plurality of atomic ridges, each of the atomic ridges including a semiconductor layer comprising Si and an dielectric layer comprising a Si compound; a plurality nanogrooves between the atomic ridges; at least one elongated molecule located in at least one of the nanogrooves; a porous gate layer located on top of the plurality of atomic ridges. The present invention also provides a membrane comprising: a substrate; and a plurality of nanowindows in the substrate and a method for forming nanowindows in a substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.09/658,599, entitled “STRONGLY TEXTURED ATOMIC RIDGE AND DOT MOSFETS,SENSORS, AND FILTERS,” filed Sep. 8, 2000, now U.S. Pat. No. 6,509,619B1, issued Jan. 21, 2003, which claims the priority of U.S. ProvisionalPatent Application No. 60/153,088, filed Sep. 9, 1999, the entiredisclosure and contents of which are hereby incorporated by reference.This application also makes reference to the following U.S. patentapplications: U.S. patent application Ser. No. 09/187,730, entitled“ATOMIC RIDGES AND TIPS,” filed Nov. 9, 1998, now U.S. Pat. No.6,667,492 B1, issued Dec. 23, 2003, U.S. patent application Ser. No.09/657,533, entitled “STRONGLY TEXTURED ATOMIC RIDGE AND DOTFABRICATION,” filed Sep. 8, 2000, now U.S. Pat. No. 6,413,880 B1, issuedJul. 2, 2002, and U.S. patent application Ser. No. 09/658,878, entitled“STRONGLY TEXTURED ATOMIC RIDGES AND TIP ARRAYS,” filed Sep. 9, 2000,now U.S. Pat. No. 6,465,782 B1 issued Oct. 15, 2002, the entiredisclosure and contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to MOSFET sensors, filters andnanostructures.

2. Description of the Prior Art

A problem with many conventional nano-sized chemical sensors is thatthey do not have a high specificity for particular chemical species tobe detected. For example, MOSFETs, or more precisely, ISFETs (IonSensitive Field Effect Transistors), may be provided with porous gatesto allow the environmental gases to arrive at the gate/dielectricinterface and modify the threshold voltage and current voltagecharacter. In fact, these ISFETs are often quite sensitive to gaseousambients of different types. However, the electrical behavior of theISFET does not generally allow different gases to be distinguished fromeach other, especially when there are several gases present in theenvironment simultaneously.

Similar to the problems faced in producing high specificity nano-sizedchemical sensors, there is also currently no good way to produce veryhigh flow-through nano-sized reaction chambers or chemical filtershaving a high degree of chemical specificity. For example, people havingan oxygen deficiency often carry around a bulky oxygen tank which mustbe refilled on a regular basis. A lightweight active filter that allowedthe passage of oxygen and very little nitrogen and larger dust andpollen particles would be a significant adjunct to the quality of lifefor many people

The present methods of electron beam lithography being developed atgreat expense may reach an ultimate minimum dimension of about 35 nm,which is indeed a large improvement over the 180 nm now being producedin the IC business. One of the best developed methods in this family iscalled SCALPEL (SCattering with Angular Limitation ProjectionElectron-beam Lithography). This system requires magnetic lenses, verythin masks, difficult mask alignment tools, and is quite complex. Itappears to have significant promise in high throughput lithography forminimum dimensions down to about 35 nm, but not for smaller dimensions.A simple contact mask for e-beam lithography producing dimensions in therange of 1 to 5 nm would be of major benefit for a wide range ofapplications.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide MOSFETsthat may be used in producing sensors having a high degree of chemicalspecificity.

It is another object of the present invention to produce nano-sizedreaction chambers having a high degree of chemical specificity.

It is yet another object of the present invention to produce nano-sizedfilters having a high degree of chemical specificity.

It is yet another object of the present invention to produce nano-sizedopenings in a contact lithographic mask and a shadow mask for charge andneutral particles.

According to first broad aspect of the present invention, there isprovided a MOSFET device comprising: a substrate including a pluralityof atomic ridges, each of the atomic ridges including a semiconductorlayer comprising Si and an dielectric layer comprising a Si compound; aplurality nanogrooves between the atomic ridges; at least one elongatedmolecule located in at least one of the nanogrooves; a porous gate layerlocated on top of the plurality of atomic ridges.

According to a second broad aspect of the present invention, there isprovided a thin membrane comprising: a substrate; and a plurality ofnanowindows in the substrate.

According to a third broad aspect of the present invention, there isprovided a method for forming nanowindows in a substrate comprising thesteps of: forming convex depressions having bottoms on a surface of a Sisubstrate; treating the bottoms of the convex depressions to formatomically flat regions; ion-implanting at least one element selectedfrom the group of elements consisting of oxygen and nitrogen into the Sisubstrate to form a lower layer comprising Si, a middle layer comprisinga Si-based insulating compound, and an upper layer comprising singlecrystalline Si; thinning the upper layer to a thickness of 5.0 to 50.0nm; depositing nanowires comprised of a first metal on the atomicallyflat regions; and etching away portions of the substrate that areunprotected by the nanowires to form a plurality of nanowindows in thesubstrate, wherein the nanowindows have a pitch of 0.94 to 5.35 nm andwindow widths of about 0.2 to 5.0 nm.

Other objects and features of the present invention will be apparentfrom the following detailed description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the accompanyingdrawings, in which:

FIG. 1 illustrates a first embodiment of the MOSFET device in crosssection of the present invention in simplified form;

FIG. 2 illustrates a second embodiment of the MOSFET device of thepresent invention in simplified form;

FIG. 3 illustrates an active membrane filter of the present invention insimplified form;

FIG. 4 is a cross-sectional illustration of an e-beam mask of thepresent invention

FIG. 5 is a simplified cross-sectional illustration of a depressionpatterning step of one embodiment of the method of the present inventionfor forming a membrane;

FIG. 6 is a simplified cross-sectional illustration of a crystalflattening step of one embodiment of the method of the present inventionfor forming a membrane;

FIG. 7 is a simplified cross-sectional illustration of oxygen andnitrogen implantation step of one embodiment of the method of thepresent invention for forming a membrane;

FIG. 8 is a simplified cross-sectional illustration of a layer thinningstep of one embodiment of the method of the present invention forforming a membrane;

FIG. 9 is a simplified cross-sectional illustration of a nanowiredeposition step of one embodiment of the method of the present inventionfor forming a membrane;

FIG. 10 is a simplified cross-sectional illustration of a nanowirethickening step of one embodiment of the method of the present inventionfor forming a membrane;

FIG. 11 is a simplified cross-sectional illustration of a coppercovering layer step of one embodiment of the method of the presentinvention for forming a membrane; and

FIG. 12 is a simplified cross-sectional illustration of a nanowindowetching step of one embodiment of the method of the present inventionfor forming a membrane.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

It is advantageous to define several terms before describing theinvention. It should be appreciated that the following definitions areused throughout this application.

Definitions

Where the definition of terms departs from the commonly used meaning ofthe term, applicant intends to utilize the definitions provided below,unless specifically indicated.

For the purposes of the present invention, the term “monolayer (ML)”refers to one atomic layer of metal on a surface of a given orientation.

For the purposes of the present invention, the term “Ultra High Vacuum(UHV)” refers to a pressure of less than 1×10⁻⁹ Torr.

For the purposes of the present invention, the term “Reactive Ion BeamEtching (RIBE)” refers to one of the plasma or dry-etching methods thatmay be used to produce the grooves of this invention.

For the purposes of the present invention, the term “surfactantrestructurant” refers to a single element or several elements that helprestructure the surface of a substrate used in the formation of grooves,ridges, tips, oxide ridges, quantum wires, or other structures of thepresent invention.

For the purposes of the present invention, the term “nanowire” refers toan overlayer row resulting from the deposition of a metal on the siliconsurface. Such a nanowire has a width of ˜1 to 4 nm, a length of 10 nm orlonger, and a pitch of ˜1 to 5 nm.

For the purposes of the present invention the term “pitch” refers to theseparation between two adjacent nanowires, atomic ridges or grooves.

For the purposes of the present invention, the term “atomic ridge”refers to a ridge formed in the silicon wafer, primarily from an etchingprocedure following the growth of nanowires.

For the purposes of the present invention, the term “Molecular BeamEpitaxy (MBE)” refers to the deposition of elements onto a substrateusing evaporators in a UHV environment.

For the purposes of the present invention, the term “nanogroove” refersto the recessed region between two adjacent atomic ridges.

For the purposes of the present invention, the term “nanowindow” refersto a nanogroove that goes all the way through a membrane, and it may bean open window or may in some cases have a thin dielectric layer in thewindow.

For the purposes of the present invention, the term “nanoborder” refersto one or more nanowires or nanoridges bordering a nanowindow.

For the purposes of the present invention, the term “long chainmolecule” refers to molecules having a length of at least 5 nm, butgenerally refers to much longer molecules or segments of very longmolecules. The term long chain molecules includes DNA, RNA,polypeptides, etc.

For the purposes of the present invention, the term “elongated molecule”includes elongated molecules such as long chain molecules, carbonnanotubes, etc.

For the purposes of the present invention, the term “etch mask” refersto any material that resists or locally slows the wet or dry (plasma)etching process.

Description

Molecules of a particular size may move into and out of gap under a“suspended” metal gate shown in FIG. 1. Different oxide modulationwidths, depths, and perhaps surface treatments of these gaps, along withdifferent metal gate porosities obtaining using different oblique angleevaporation, along with nanotubes and other long molecules or segmentsthereof, may give a very large amount of specificity to the MOSFETdevice character for gaseous and liquid detection. Good specificity isvery important for chemical sensors, and the concept of gangs ofdifferent length MOSFETs having different width, depth and gapcharacteristics may be used to analyze complex mixtures of chemicals. AMOSFET of the present invention includes a porous metal gate with poresthat have similar but not identical diameters. The pore size may becontrolled over a wide range by varying the angle of obliquity of theevaporant, the temperature of the receiving substrate, the rate ofevaporation, and the rate of rotation of the substrate during thedeposition process. See K. Robbie, et al, J. Vac. Sci. Tech. B, 16(3),1115–1122 (1998) for experimental data on the porosity of thin filmsevaporated under a wide range of conditions.

A gated single walled nanotube (SWNT) of carbon supported on an SiO₂substrate has been shown to be extremely sensitive to NH₃ and NO₂ gasmolecules, see Kong et al. “Nanotube molecular nanowires as chemicalsensors”, in Science, 287, 622–625 (2000), the entire disclosure andcontents of which is hereby incorporated by reference. Two and threemagnitude changes in the current of a MOSFET-like structure wereobtained when the nanotube was exposed to these gases while usingdifferent “gate voltages” obtained by biasing the substrate relative tothe source and drain metal nanowires on the covering oxide. The presentinvention provides a ridged or dot MOSFET having a suspended porous gatewhich may have the effective work function of the gate modified with gasmolecules or with SWNTs stretched out by the millions in the undulationsof the oxide.

The atomic ridges of the present invention may be formed by anyappropriate process for forming atomic ridges described in the 1998 U.S.patent application Ser. No. 09/187,730 entitled “Quantum Ridges andTips” and in the concurrently filed U.S. patent application entitled“Strongly Textured Atomic Ridge and Dot Fabrication”. Both of theseapplications list Don L. Kendall as an inventor and the contents anddisclosure of both these applications are hereby incorporated byreference.

In one preferred method for forming atomic ridges of the presentinvention, a simple oblique incidence evaporation of an etch resistantmetal onto a pristine silicon surface under Ultra High Vacuum UHVconditions is conducted. The oblique incidence evaporation is thenfollowed by a brief etching step (dry or wet). Other single crystalmaterials other than silicon may also be used for the substrate, forexample, Ge, diamond, and the III–V compounds, as well as othercompounds and even metal crystals.

For preparing atomic ridge products of the present invention, asubstrate, preferably a semiconductor wafer, most preferably a circularSi (1 1 X) wafer, is prepared by standard chem-mechanical polishingmethods. When the substrate is a silicon wafer, the substrate ispreferably heated in a UHV chamber at a pressure of about 10⁻¹⁰ Torr toa temperature of 1150° C. for a brief period (“flashed”) to remove anysurface oxides and then cooled to below room temperature (around −20 to25° C.). The heating of the wafer may also be accomplished locally usinga focused or beam-expanded laser passing through a quartz window in themolecular beam epitaxy (MBE) system. This heating process leaves thesurface in a stable condition with slightly elevated parallel atomicridges or misalignment steps having a pitch of 0.54 to 60 nm, mostpreferably 0.94 to 5.4 nm for surfaces of (1 1 2) to (5 5 12),respectively. The atomic ridges may have occasional atomic steps in themalong their length due to the slight variations from perfect flatness ofthe wafer surface, but after each misalignment step, the ridges againestablish themselves in the same <1 1 0> direction. In addition, theremay be occasional reconstruction faults in the surface, especially onthe (5 5 12) surface. By contrast the (1 1 4), (7 7 15), and (5 5 11)surfaces are generally completely free of restructuring faults.

The substrate is then coated to form quantum wires on the ridges. Apreferred technique for forming quantum wires is to use obliqueevaporation at a small angle of 1 to 5° (or up to 30 degrees iseffective in some cases) with an etch resistant (or in certain quantumwire applications “conductive”) metal such as Au or Cr, or Al or Be sothat the slightly higher (by about 3 Å) ridges are coated preferentiallywith 5 to 30 Å of the metal relative to the intervening very shallowtrough of the restructured clean surface. Preferably, the substrate isrotated during this process while maintaining the obliquity to improvethe uniformity of coverage along the ridges. This rotation also helps toavoid bridging of the metal due to the “lateral needles” that form whenevaporating at high obliquity. This rotation may be modified by blockingoff the evaporating beam with a raised barrier on the sample holder oron the wafer itself along the direction of the atomic ridges so that theevaporation source never is in direct line with the atomic troughs.

An alternative method for forming atomic ridges for use in the presentinvention involves depositing nanowires of Ag or Au on a Si substrate.This process preferably starts with a high temperature, on the order of1100–1200° C., treatment of a special orientation wafer such as Si(7 715), Si(5 5 11) or Si(5 5 12) in an Ultra High Vacuum (UHV) system. Thewafer is then cooled at a slow rate to near room temperature (RT), atwhich point a fraction of an atomic monolayer, ML, of an element such asAu, Ag, Ga, etc. is evaporated onto the wafer. The wafer is then heatedto a specific temperature range, depending on the element deposited, thethickness of the element to be deposited, and the substrate on which theelement is deposited. As a result of the heat treatment, nanowires areformed of the deposited element.

After the metal nanowires are formed on an Si wafer, additionaltreatments to the Si wafer are used to produce grooves in theunprotected Si between the metal nanowires, thereby producing atomicridges. Depending on the application, the metal nanowires may be removedfrom the atomic ridges.

In one preferred embodiment, the additional treatment of the Si waferincludes thickening or protecting the thin metal nanowires to make themetal nanowires more etch resistant during various conventional wetchemical or dry etching treatments. Once the metal nanowires have beenthickened, the Si wafer is etched to produce grooves having a pitch of0.94 to 5.35 nm.

A method of depositing Ag on a Si(5 5 12) surface suitable for use inthe method of the present invention is described in Baski et al., J.Vac. and Tech. 17, 1696–1699 (1999), the entire disclosure and contentsof which is hereby incorporated by reference. The process of Baski etal. starts with a clean well-polished single crystal wafer of Si(5 512). The wafer is then heated to 1150–1200° C. in a UHV system operatingat a pressure of about 10⁻¹⁰ torr to vaporize or flash any native oxideand to allow the surface to restructure into the well-ordered structurecharacterized by a surface unit cell of 5.35 nm. The wafer is thenslowly cooled to room temperature, and very thin layer of only about0.25 monolayers (ML) of Ag is evaporated from a tungsten filament onto awafer. The sample is then heated at a pressure of about 10⁻¹⁰ torr toabout 450° C., at which temperature the Ag atoms move around on thesurface and form Ag nanowires of about 1.6 nm width on a pitch of 5.35nm, namely on the spacing of the surface cell mentioned above. This issimilar to the process for the deposition of Au on such as surfacedescribed in U.S. patent application Ser. No. 09/187,730 filed Nov. 9,1999, the entire contents and disclosure of which is hereby incorporatedby reference.

In the method of the present invention, when Ag nanowires are deposited,preferably the Ag is deposited to a thickness of 0.15 to 2.5 ML on a Sisubstrate. In the method of the present invention, when Au inducedrestructuring is desired, preferably the Au is deposited to a thicknessof 0.04 to 1.0 ML on a Si substrate.

When silver is the metal deposited as nanowires, the deposited silvernanowires may be used as etch masks for etching Si, either with wetchemistry or dry etching. Preferably, the Ag nanowires are thickened bydepositing more Ag or Au, or other materials to make the nanowires morerobust etch mask. When the Ag nanowires are thickened with Ag, theresulting thickened nanowires preferably have a thickness of about 3 to5 ML. When the Ag nanowires are thickened with Au, the resultingthickened nanowires preferably also have a thickness of about 3 to 5 ML,although thicknesses up to at least 20 ML are possible for both Au andAg thickening using oblique evaporation along with a rotating substrate.A Cu or Au-coating completely covering the Ag nanowires allows UHVsamples to be taken out into normal room ambient temperature forsubsequent processing.

Without pre-stressing of the substrate on which Ag is deposited, theremay be occasional surface faults that disrupt the regularity of theatomic ridges for about 20% of the ridges. Occasionally there are extra(3 3 7) segments of 1.6 nm width on the (5 5 12) structure, so thesequence may become something like: 5.4 nm, 5.4 nm, 5.4 nm, 5.4 nm, 5.4nm, 7.0 nm, 5.4 nm, 5.4 nm, etc. Alternatively, occasionally there aremissing 1.6 nm segment so that the sequence may become something like:5.4 nm, 5.4 nm, 5.4 nm, 5.4 nm, 5.4 nm, 3.8 nm, 5.4 nm, 5.4 nm, etc.However, typically, a sample either has all extra 1.6 nm faults, or ithas all missing 1.6 nm segments. This is highly suggestive that mountingsamples under compressive stress, or alternatively, tensile stress,during the passage of heating current to raise the samples to 1100 to1200° C. may remove these faults or control the density of these faults.

Depending on the application, the faults in the Si wafer may becontrolled or removed using compression or tension. For example, thesurface faults may provide controlled disruption of the Bragg-Lawreflections, which provides plateaus that may be desirable in theproduction of MOSFETs. For other applications, very light compression ortension may be applied to the Si wafer, to eliminate surface faultscompletely to get large regions of 5.4 nm sequences. One method ofcontrolling such tension or stress involves using quartz wedges duringthe high temperature flashing process and/or at lower temperatures.Another way to provide a stress-free Si(5 5 12) surface is to float anSi wafer on a molten tin cushion during an MBE/UHV treatment.

The problem of small modulations on the surface of the Si wafer may beaddressed in one of at least two ways. One way of solving the problem ofsmall surface modulations is to add concave or convex regions on the (55 12) or other (1 1 X) wafers, as described in U.S. patent applicationSer. No. 09/187,730, the entire disclosure and contents of which ishereby incorporated by reference.

However, the step of forming concave or convex regions may be avoidedfor many applications by cutting and polishing the Si(1 1 X) waferoff-axis in particular directions by about 0.5 to 4 degrees. Preferablythe angle chosen is larger than the largest local Si wafer undulationangles, thereby ensuring that the misalignment steps will be in the samegeneral directions, though not very regularly spaced nor straightbecause of the undulations. As Si wafer local flatness improves and asthe precision of the original orientation angle improves, the need foroff-axis cuts of the Si wafer diminishes.

It should also be noted that purposeful misalignment by less than 0.5degrees, down to about 0.1 degrees, may also be useful in someapplications, since this will result in single monolayer steps. Singlemonolayer steps may provide greatly different kinetics, compared to thedouble steps of wafer misorientations greater than about 1.0 degrees,during various processing steps. Crystal cleavage on a perfect crystalplane may be used to eliminate these steps, but this does not occurexcept on the (1 1 1) or (1 1 0) planes, and doing so on large wafersmay be difficult.

An important additional process step may be taken after the original Sietch step of only a few nanometers depth, either using RIBE or wetetching. This additional process step involves an oblique evaporation ona rotating substrate in order to build up a thicker metal or dielectricetch mask of several monolayers. After the additional monolayers aredeposited, a second RIBE or other dry etching step may be performed inorder to produce significantly deeper grooves of 2.0 to 20.0 nm depth,or even much deeper when the evaporated film is much thicker. Thisoblique evaporation may also be done on the original Ag depositedsurface or even on the surface of a Si(1 1 X) before it has been throughthe Ag deposition process. However, the original surface topology of 0.2to 0.3 nm, without the Ag, is quite shallow, and a preferred method isto increase the topology somewhat before the oblique evaporation step.These thicker grooves allow a much wider range of applications.

A fraction of a monolayer (ML) of an element like Ga, asurfactant-restructurant may be applied to the surface of the Si waferprior to deposition of metal atomic strips to encourage elements such asAu or Ag deposited on the Si wafer to rapidly move to the desired atomicridge positions.

Depositing a little less than one monolayer of Ga on a (1 1 2) Si wafermay force the Si wafer to favor the (1 1 2) facet formation after a 500°C. treatment rather than a mixture of (3 3 7) and (1 1 1) sawtoothfacets that are typically observed on a clean (1 1 2) surface. A (1 1 2)structure generally provides the smallest pitch in the (1 1 X) family inwhich the nanogroove walls are physically robust. Such reconstructionsare described in Baski et al., “The structure of Si(1 1 2)-Ga(Nx1)reconstructions”, Surf. Sci. 423, L265–270 (1999), the entire contentsand disclosure of which is hereby incorporated by reference.

Using 0.04 to 0.12 ML of Au on slightly irregular (5 5 12) shows that Si(5 5 12) surface restructures to many (7 7 15) facets with interposed (11 3) steps after heating the wafer at about 800° C. The (7 7 15) atomicridges have a very regular spacing of 3.45 nm when the Au is present.The situation with (7 7 15) atomic ridges is unlike the situation with(5 5 12) atomic ridges which may as have as many as 20% surface faults.Therefore, precisely aligned (7 7 15) wafers may be produced by Simanufacturing processes and then an 800° C. Au treatment may be appliedto obtain large areas of well structured material with very regularspacing of 3.45 nm. To utilize this regular spacing to produce regularlyspaced nanogrooves of 3.45 nm pitch may be accomplished by usingsubsequent oblique evaporation, an electrochemical process, etc. of ametal or other etch mask and a dry or wet etch process to produce thegrooves. The above-described restructuring process involves only asingle impurity.

Small amounts of either Ga, or even Hg or Sn, used in conjunction withAg, Au, or Al may also be useful as surfactants. The use of these smallquantities of surfactants may lower or broaden the useful heat treatmentrange, and may help heal irregularities.

An additional aspect of using surfactants like Au at 800° C. or Ga at500° C., or perhaps Hg at a much lower temperature is to clean orrestructure a surface prior to an MBE process at a similar or highertemperature. H₂, O₂ and cracked H₂ may be used for removing nanowires ofAg, Au, Ga etc. from the atomic ridges so they may be used to formMOSFETS.

Nanowires of Ag, Au, Ga, etc. may also be removed form the atomic ridgesby wet chemical cleaning using standard processes.

To form a MOSFET of the present invention, an upper dielectric oroxidized layer is formed in a substrate including atomic ridges byheating the wafer in an oxidizing or nitriding atmosphere.

After the dielectric layer is formed on the surface of the substrate,long chain molecules are deposited in nanogrooves between the atomicridges by any of several methods. For example, the molecules may besuspended in a solvent and allowed to settle into the nanogrooves,perhaps in an electric field to stretch out the molecules in the generaldirection of the nanogrooves.

To form a MOSFET, after the long chain molecules are placed in thenanogrooves, a porous gate layer comprising an obliquely evaporatedmetal like Al, Au, Cu, or W is deposited on the wafer. This metal gatelayer rides across the top of the atomic ridges to form enclosed regionsthat contain the long chain molecules. Pores in the porous gate layerallow molecules to enter the MOSFET and be adsorbed or chemicallyreacted with the long chain molecules in the enclosed regions. Methodsof forming MOSFETs are well known in the art, and are described, forexample, in U.S. Pat. No. 6,667,492, incorporated above by reference.

FIG. 1 illustrates a section of one embodiment of a MOSFET device 102 ofthe present invention. MOSFET device 102 includes a substrate 104including an Si layer 106, and SiO₂ dielectric layer 108, a long chainmolecule 110, a porous gate layer 112 and an enclosed region 114. Largepores 116 allow large molecules 118, typically 0.7 to 0.9 nm in averagediameter to enter enclosed region 114 and be detected by their effect onthe conductance of the long chain molecules 110.

FIG. 2 illustrates a section of second embodiment of a MOSFET device 202of the present invention. MOSFET device 202 includes a substrate 204including an Si layer 206, and SiO₂ dielectric layer 208, a long chainmolecule 210, a porous gate layer 212 and an enclosed region 214. Smallpores 216 allow small molecules 218, typically 0.3 to 0.6 nm in averagediameter to enter enclosed region 212 and be detected by their effect onthe long chain molecules 210.

A MOSFET of the type shown in either FIG. 1 or FIG. 2 is an excellentstructure for sensing gases, as well as ions in liquids.

The spacing between atomic ridges may be 1.63 nm for (1 1 4) surfaces ofSi, or it may be 5.4 nm for (5 5 12), as well as many other spacings forother (1 1 X) surfaces. The (5 5 12) surfaces provide larger groovewidths, 1.6 to 3.2 nm depending on the depth of the undulations and theoxidation or deposition conditions, that are easily capable ofcontaining the 1.6 to 1.8 nm diameter nanotubes. The oxide thickness maybe adjusted to detect different size and shaped molecules.

It is important to note that the MOSFET device of the present inventionis similar to a standard MOSFET, except for the STAR ridges, the porousgate, and the deposition in the oxide grooves of nanotubes or other longchain molecules. The current may flow laterally from left to rightacross the inversion layers of FIG. 1, or it may be arranged to flowinto the plane of FIG. 1 by changing the positions of the sources anddrains.

The present invention also provides nano-sized reaction chambers forcarrying out chemical reactions, such as the reactions that may takeplace in an active filter. For example, a thin segment of a (5 5 12) orother (1 1 X) Si wafer may be used to form water from even very dry airusing the small mole fraction of about 5×10−⁶ of H₂ gas in theatmosphere. This H₂ gas is allowed to pass through a narrow nanowindowin the filter and O₂ is allowed to pass, and perhaps dissociate due to aPt catalyst on the walls of the filter, through a wider neighboring gap.Then the H₂ and the excited oxygen atoms recombine on the downside ofthe filter to form water, as shown in FIG. 3. Nanowindows of differentwidths may be formed on Si(5 5 12) by near grazing incidence evaporationonto the tops of the pi-chain ridges that make up the (5 5 12) unitcell. These atomic ridges have spacings of 1.6, 1.6, and 2.2 nm withinthe 5.4 nm unit cells of the (5 5 12). These spacings provide twodifferent spacings of the nanoridges of 1.6 and 2.2 nm. The groovesbetween these ridges are typically 0.6 smaller than the spacings of thenanoridges, so neighboring nanowindows are about 1.0 and 1.6 nm wideafter the dry etching process, as shown in FIG. 3. However, if narrowernanowindows are desired to encourage particular chemical reactions, theridges may be oxidized and the resulting window widths will be about 0.4and 1.0 nm wide. Many other options exist using these generalprinciples.

The active filter shown in FIG. 3 may be used for oxygen enrichment ofnormal air. For example, if the O₂ dissociates in a small groove perhapsdue to the presence of a catalyst such as Pt on the walls of thegrooves, as shown in FIG. 3 and the N₂ in the air sample does notdissociate, then the O₂ will effectively move much more efficientlythrough the filter since it is much smaller while it is passing throughthe structure, but it almost instantly recombines on the downside of theactive filter. Such an active filter may be used in a mask for placingon an individual's face. Alternatively, the filter may be used in someother oxygen delivery system to replace the large oxygen tank typicallyused by individuals with oxygen deficits.

To prevent H₂O vapor from plugging the active filter of the presentinvention, the formation of SiO₂ may be inhibited by using anH-passivation process on the Si filter. One method of H-passivation isto soak the filter in a dilute solution of HF, preferably stabilizedwith F-compounds and/or by Pt or other catalyst(s).

The active filter may also be used to form H₂ by passing normal watervapor form the atmosphere through the grooves and forming H₂ and perhapssinglet excited oxygen on the down side using a suitable catalyst on thewalls of the filter, as shown in FIG. 3. The H₂ so formed may be used ina fuel cell without the need for any bulky H₂ storage process. Apreferred catalyst is Pt.

H₂ may also be formed by the diffusion of singlet H⁺ ions from a denseplasma through the active filter of FIG. 3 by the reaction H⁺+H⁺+2e⁻→H₂.This reaction proceeds extremely rapidly, with an estimated turnoverrate of about 2×10⁵ molecules/sec inside a filter containing singlechannel wall thicknesses of about 0.3 nm (rather than the double channelwall thicknesses of 0.6 nm of the oxygen enriching filter.) Thisturnover rate is faster than most inorganic catalytic process by afactor of 1000 or so, and is within a factor of 5 of perhaps the fastestrates known, namely the hydgrogenase reactions for converting H⁺ ionsinto molecular H₂ in the human body. This very fast turnover rate is dueto the extremely thin vertical membranes on the (1 1 2) or other (1 1 X)surfaces in conjunction with the rapid diffusion of H in Si. A Pt orother catalyst on the surface of the vertical membranes may be used toensure rapid recombination of H ions (protons) to form molecularhydrogen. Because two electrons must be extracted from the Si for everymolecule of H₂ that is formed, a very large current density of about 56A/cm² results from this process. This large current flow may inprinciple be drained off the membrane to ground as the plasma movesthrough the membrane.

The present invention also provides a nanometer size electron beamlitho-mask. Lithographic processes currently being developed, such asSCALPEL (scattering with angular limitation projection electron-beamlithography), may have an ultimate minimum dimension of 35 nm. Incontrast, the litho-mask of the present invention may have a resolutionof a small as 2 nm with a pitch at least as small as 5 nm. FIG. 4illustrates a litho mask that may be used as a contact mask for e-beamlithography.

The litho masks or membranes of the present invention may be made in aseries of steps. First, depression patterning is performed on thesubstrate, which preferably comprises Si. Light assisted chemical etch(LACE) process, an E-beam process, or Focused Ion Beam/Gas AssistedEtching (FIB/GAE), may be performed to produce convex regions at desiredlocations in the depression patterning process. Depression patterningmay also be done using a chemical etching process along the linesdescribed in U.S. patent application Ser. No. 09/187,730, the entirecontents and disclosure of which is hereby incorporated by reference.Any of these localized etching process may be followed by a briefChem-Mechanical Polish (CMP) touch up process, if desired. FIG. 5illustrates a depression patterning step of one embodiment of thepresent invention.

The convex regions of the treated substrate may then be conditioned tobe atomically flat (1 1 X) over tens of microns using Ar ion bombardmentat 800 to 975° C. for 1 to 5 minutes, or by allowing molecular oxygen toimpinge on the surface at a pressure of about 4×10⁻⁸ torr at 800 to 975C for 1 to 5 minutes; see J. B. Hannon, et al, Phys. Rev. Lett.81,4676–4681 (1998). Another way to condition the convex regions to makethem atomically flat is to heat the substrate in ultrahigh vacuum (UHV)at about 1150 to 1200° C. for several hours; see s. Tanaka, et al, Appl.Phys. Lett. 69, 1235–1237 (1996). However, using a process having anoperating temperature less than 975° C. is preferred in most cases toavoid plastic deformation of Si. FIG. 6 illustrates such a conditioning(atomic flattening) step of one embodiment of the present invention.

In order to make a very thin e-beam mask or membrane, a low energyimplantation of oxygen, nitrogen, or a mixture of oxygen and nitrogen isconducted on the substrate. This SIMNOX or SIMOX treatment is preferablyconducted at an elevated temperature at a dose of about 10⁻¹⁸/cm². Thesubstrate is then heated to a still higher temperature, such as 1050 to1200° C. to form the oxide, nitride, or oxy-nitride under the surface ofthe (1 1 X) Si, while leaving the top thin layer single crystalline,thereby forming a multi-layer wafer. One advantage of the “Separation byImplantation with Nitrogen and Oxygen” (SIMNOX) using silicon nitride orsilicon oxynitride is that the underlying layer will be in tension tocause the ultimate ultra-thin mask to be stretched flat rather than beseverely wrinkled as it would be with a silicon oxide (SIMOX) film. FIG.7 illustrates the results of an N and O implantation step of oneembodiment of the present invention. An early description of theso-called SIMOX or SIMNOX process for producing either an oxide or anoxynitride layer, respectively, under a single crystal Si layer is U.S.Pat. No. 3,807,274 to Kendall et al., the entire contents and disclosureof which is hereby incorporated by reference.

It should be noted that the SIMNOX or SIMOX treatment described abovemay be performed before depression patterning with similar results. Infact, the top of the dielectric film produced may be somewhat smootherif the implantation is done before depression patterning.

The single crystal layer produced by the implantation process describedabove will preferably have a thickness of about 0.1 to 0.5 μm. Toproduce particularly thin grooves from this single crystal layer, theupper layer of Si must be thinned down to about 5.0 to 50.0 nm. Thisthinning may be accomplished by oxidation in O₂ at a temperature of 800to 900° C. followed by dipping of the multi-layer wafer in dilute HF orBuffered Oxide Etch (BOE). The thinning of the upper Si layer may alsobe accomplished by either wet or dry etching. However, oxidation andanodization processes are preferred to provide more uniform layerremoval. FIG. 8 illustrates the results of a thinning step of oneembodiment of the present invention.

The process of thinning the substrate may be performed prior todepression patterning if implantation of oxygen, nitrogen or a mixturethereof is conducted prior to depression patterning.

Ag nanowires is then deposited under UHV on the (5 5 12) multi-layerwafer with 5.4 nm pitch in 1.6 nm wide parallel nanowires. The processof deposition preferably includes depositing Ag nanowires having athickness of 0.25 ML to 2.5 ML at a temperature of 20° C. up to 150° C.After deposition, a heat treatment step is performed in which themulti-layer wafer with the very thin layer of deposited Ag is heated ata temperature up to 450° C. FIG. 9 illustrates the results of a nanowiredeposition step of one embodiment of the present invention, where thevertical scale has been increased slightly for clarity.

If a thicker more robust membrane or etch mask is required, a Audeposition of 0.20 to 5 ML or more may be applied at a temperature of200 to 350° C. The Au will have a strong tendency to collect only at theposition of the Ag strips due to Au's high surface diffusion coefficienton the Si surface, and, possibly due to the less-than-unity stickingcoefficient for Au at 200 to 350° C. FIG. 10 illustrates this Authickening step of one embodiment of the present invention.

If the multi-layer wafer of the present invention with deposited Agnanowires or thickened Au/Ag nanowires is to be removed from the vacuum,a 5.0 to 50.0 nm thick layer of Cu may be evaporated, preferably at nearnormal incidence, on top of either the Ag nanowires or the thickenedAu/Ag nanowires, respectively, without breaking the vacuum of the UHVsystem. Coating the nanowires with Cu allows for the samples to beremoved form the UHV system and stored at room temperature for longperiods of time without oxidation. Such a Cu coating also makes thenanowires resistant to airborne and liquid particle attachment. The Cumay be readily removed using dilute HCl or other known solvents for Cuthat will not attack the Au or Ag layers. FIG. 11 illustrates a Cucovering layer step of one embodiment of the present invention.

The resulting nanowire multi-layer wafers may then be dry etched, wetetched, chemically etched, or electrochemically etched to providenanowindows in a thin membrane as shown in FIG. 12. In the embodimentshown in FIG. 12, the nanowindows are surrounded by Au—Ag—Sinanoborders. However, if the Ag nanowires are not coated with Au duringprocessing, the nanoborders would be Ag—Si nanoborders.

An alternative way to produce a thicker membrane for use as an etch-maskis to evaporate a metal or dielectric film at near-grazing incidence,which will provide an oblique evaporation thickening. If this is done ona rotating substrate that has an existing ridged surface morphologysimilar to that produced in the methods for forming a mask describedabove, a distinct columnar structure will be produced.

Vertical columns or ridges at least 10 times as high as the thickness ofthe Si wafer upon which the columns are deposited by oblique evaporationthickening may be obtained, as shown by Hannon, et al mentioned earlier.So, for example, a 3.4 nm high ridge on a Si (5 5 12) wafer will supporta ridge or column of a deposited material at least 34.0 nm high. Toremove irregularities, after the oblique evaporation thickening, the Siwafer with thickened ridges or columns may be dipped in a basic solutionsuch as KOH:water or an organic base to straighten out anyirregularities in the bombarded walls caused by warped etch masks in thebombarded walls.

Due to the thinness on the etch mask produced by the process of thepresent invention and the nanowindows therein, the etch mask andnanowindows of the present invention are preferably provided with etchedhoneycomb supports, such as those described in the U.S. Pat. No.3,936,329 to Kendall et al., the entire disclosure and contents of whichis hereby incorporated by reference.

The etch mask of the present invention may be used for a near-contacte-beam mask with very small patterns. The etch mask of the presentinvention may also be used for shadow masks for different angleevaporation or different types of energetic particles. In addition, theetch mask of the present invention may used for making MOSFETs withnanometer size features or for making SCALPEL masks more robust and/ormore tolerant of angular deviations in an e-beam. The etch mask of thepresent invention may also be used to make a slightly diverging orconverging e-beam into one or more parallel beams.

A surprising and potentially useful feature of the thin membranes of thepresent invention is that they may be produced by this technology isthat the membranes may be heated and cooled in incredibly short times.An estimate of the ultimate cooling time (and heating time) is given bythe equation: τ_(c)=C_(p)d²/K, where C_(p) is the specific heat, d isthe thickness, and K is the thermal conductivity of the membranematerial. Using the known values of these parameters for Si, andassuming a thickness of 1.0 μm for simplicity, the ultimate cooling rateis calculated as 70 ns. For the more relevant thinnest membrane aboveused in this invention of 5 nm, the ultimate cooling rate is about2×10⁻¹² s, which is much faster than any switching time of a device in atypical IC. This means that a thin membrane is very sensitive to currentand voltage variations while operating as an environmental or flowsensor.

Although the present invention has been fully described in conjunctionwith the preferred embodiment thereof with reference to the accompanyingdrawings, it is to be understood that various changes and modificationsmay be apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims, unless they departtherefrom.

1. A MOSFET device comprising: a substrate including a plurality ofatomic ridges, each of said atomic ridges including a semiconductorlayer comprising Si and a dielectric layer comprising a Si compound; aplurality nanogrooves between said atomic ridges; at least one elongatedmolecule located in at least one of said nanogrooves; a porous gatelayer located on top of said plurality of atomic ridges.
 2. The MOSFETdevice of claim 1, wherein said atomic ridges have a pitch of about 0.94to 5.4 nm.
 3. The MOSFET device of claim 1, wherein said dielectriclayer comprises SiO₂.
 4. The MOSFET device of claim 1, wherein said atleast one elongated molecule comprises a long chain molecule.
 5. TheMOSFET device of claim 1, wherein said at least one elongated moleculecomprises a carbon nanotube.
 6. The MOSFET device of claim 5, whereinsaid MOSFET device includes detecting means for detecting the presenceof NH₃ and NO₂ gas molecules and wherein said detecting means comprisessaid nanotubes.
 7. The MOSFET device of claim 1, wherein said at leastone elongated molecule comprises a plurality of elongated moleculeslocated in said plurality of nanogrooves.
 8. The MOSFET device of claim1, wherein said porous gate layer has pores having diameters of 0.8 to4.0 nm.
 9. The MOSFET device of claim 8, wherein said porous gate layercomprises a high conductance metal.
 10. The MOSFET device of claim 1,wherein said porous gate layer has pores having diameter of 0.3 to 0.7nm.
 11. The MOSFET device of claim 10, wherein said porous gate layercomprises Au.
 12. The MOSFET device of claim 1, wherein said porous gatelayer has pores that all are the same diameter to a tolerance of 0.2 nm.13. The MOSFET device of claim 1, wherein said porous gate layerincludes pores that differ in diameter by at least 0.7 nm.